Contrast ratio vs black level

I occasionally peruse Internet A/V forums to see what the techier web denizens have to say about the latest news and reviews.

One thing I've noticed a lot of lately, especially after our big projector 3fer, is a fixation on black level, with no mention or thought about contrast ratio.

This is a big deal, as black level without contrast ratio can result in some pretty terrible picture quality.

First, a jargon primer. Black level is the darkest image a display can produce. Think of an image of a night sky (hey, there's one right up there!). How dark is the black parts of that black sky? Different display technologies produce this "black" with different levels of success.

If you were watching an OLEDTV, the black would be actual black: as in the absence of light. With one exception, no other display technology can do this.

Plasma televisions, though excellent on the contrast ratio front (more on that in moment), still produce some light when they're supposed to be "black." They've gotten really close to black though, and this year's models promise even better performance.

The "liquid crystal" part of LCD TVs is merely a valve for the light created by the backlight. The backlight (be it CCFL or LED) always creates light, and the liquid crystal regulates this to produce an image. Some LED LCDs can adjust areas of their backlights (called "local dimming") to dim parts of the screen that should be dark, but these have can have some side effects which we'll discuss below. There are a tiny handful of full local dimming backlights that can adjust the levels of their LEDs in fairly small areas of the screen, but these are extremely rare. For more on the distinction between all of these, check out LED LCD Backlights explained.

With projectors, the most common way to improve black level is with some kind of iris. During dark scenes, the iris closes, blocking the amount of light on screen and making it appear darker.

But What About Contrast Ratio?

OK, so that's black level. Contrast Ratio, on the other hand, is the difference between the darkest part of the image, and the brightest. Remember that night sky example before? How dark is the sky, versus how bright are the stars. As in, how dark is the black, compared to how bright is the white? This is contrast ratio, or at least, it used to be.

Manufacturers, for years, have claimed outrageous (and flat-out untrue) contrast ratio numbers. Most are completely made up. Those that have any basis in truth, are what's called "dynamic" measurements. Take, for example, the projector iris. Remember how it closes to make the image darker? Well that also darkens the bright parts of the image. So the whole thing is dimmer. Yes, the black level is "better" but at the expense of making the bright parts of the image darker too. The native contrast ratio doesn't change, the whole thing just gets darker. A dynamic contrast ratio measurement uses the peak brightest white (iris open) to the darkest black (iris closed). The thing is, these two things can't happen at the same time.

With local dimming LCDs, this is basically the same issue. The LEDs can dim, but this also reduces how bright the bright parts are.

With a fast-moving iris, or a well designed locally dimming LCD backlight, the result is definitely better than the same display without either feature engaged (relying, instead, on the TV/projector's native contrast ratio alone). However, this isn't as good as a good native contrast ratio.

In real life, bright objects are bright, dark objects are dark, and our eye is able to distinguish these to a great degree. A TV/projector with low contrast ratio will look washed out, flat, boring. A high native contrast ratio will really pop, the image taking on an almost 3-dimensional quality, even when it's just 2D. A high dynamic contrast ratio is somewhere in between, at least the better ones.

Barring other factors like color or color temperature accuracy, a display with a high native contrast ratio is going to look better than a display with a low native contrast ratio, even if it has a good dynamic contrast ratio.

The best two displays I measured last year were not coincidentally those with the highest native contrast ratios. We awarded the Panasonic TC-P55VT50 a Video Product of the Year Award. The other, the JVC DLA-X35, is one of my favorite projectors ever. Both had a significantly better contrast ratio than any other displays in their categories.

So what about black level for the sake of black level? If a display doesn't have a good native contrast ratio, and you force it to produce a dark black level, the image is going to be dim and lifeless. This is because the only way it can produce dark blacks is by also producing dark whites. Not ideal.